Gravity meter



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@L9 23 l @M uw ff M www Patented ug. A8, 1944 UNITED stares mirenGRAVITY METER Daytonlil. Clewell and Henry A. Maeder, Dallas,

Tex., assignors to Socially-Vacuum Oil Company, Incorporated, New York,N. Y., a corporation of New York Application August 6, i941, Serial No.405,634

17 Claims.

This invention relates to instruments for `measuring the force ofgravity and more particularly to an instrument of this type that issensitive enough and rugged enough to be used in the field work ofgeophysical exploration, In' this Work comparisons of gravity.measurements made at many points on an area of the earths surface arecompared and the information so obtained utilized as an indication ofabnormalities in subsurface structure.

a small change in' angle would have a greater effect upon the action ofgravity on the mass.

' To still further avoid errors that might'be introduced by the massbeing at a different position when different measurements are taken orthe parts of the meter being differently disposed about the pivot, anarrangement has been incorporated in the new gravity meter whereby allGravity meters have long been used and many types have been devised forthe particular purpose of geophysical exploration, but none has beenentirely satisfactory. -Among the common faults with present day gravitymeters, are lack of sensitivity, sensitivity to forces other than theearths gravitation, excessive Weight, lack of ruggedness, difficulty ofadjustment, long periods necessary for operation and lack of propertemperature and -barometric compensation.

- so that variations of temperature andbarometric pressure havesubstantially no effect on.it 'and possesses other marked advantagesover the gravity meters in the prior art.

The new gravity meter is of the type generally known as .the horizontal,pivoted beam type. This means that amassis carried at one end of asubstantially horizontal be'am pivoted at the other end through a fixedsupport. The beam is prevented from rotating to any great extent aboutthe pivot by a spring or other resilient support for the mass. This typeof gravity meter, which is known, has been found to be particularlyadvantageous -because the force of gravity acts in the direction oftravel of the mass and the mass is prevented from. travelling in anyother direction y by the pivot. Thus, stray forces acting inI directionsother than that of gravity are to a great extent prevented fromaffecting .the mass. Furthermore, since the mass is travelling in adirection at a greater angle to the force of gravity measurements of.gravity will be taken with the mass in exactly thesame position, thatis to say, a null system has been devised whereby the force necessary tobring the gravity meter mass back to a zero position is read as themeasurement of gravity.

In addition to this the device lincludes an arrangement of partssuspended on a single support in such a way that force applied to theoutside casing will not warp the parts and thus produce inaccuratereadingsv of the` gravity meter. It has been found that even thetightening or loosening of a screw in a very heavy case Will causeserious errors if the 4parts of the gravity meter are supported directlyon the case.

Stillfurther the present gravity meter includes means for clamping themoving parts in vposition and releasing these parts at a later timefor-operation, without disturbing the calibration or causing a swingingof the gravity meter mass to such an extent that the making of a readingwill be materially delayed. Cooperating to this same end, damping meanshave been provided to damp the movement of the gravity meter mass inal1-directions so that the mass comes quickly and ,easily to rest in aposition where it can be accurately read.

In addition to these features means are provided for maintaining theWorking parts of the gravity meter at a constant temperature and forcompensating for variations in barometric pressure. Means for making allnecessary adjustments simplyA and expediently are also incorporated inthe new device.

Still further, the device has been carefully arranged to prevent anyvibration of the resilient supporting means for the mass so that thegravity meter may be moved around and handled 'in a relatively rough andcareless manner without sustaining any injury or affecting itscalibration.

vsubstantially vertical direction, small errors in Many furtheradvantages and details of this invention will be apparent from thefollowing description of the preferred embodiment thereof. It is to beunderstood that anyor all of the features incorporated in this preferredembodiment may Ibe incorporated in the gravity meter made according tothis vinvention and that it is in disclosed, insofar as those featuresare patentable. l

In the drawings: l. j Figure 1 is a diagram developing the theory of thehorizontal pivoted beam gravity meter;

Figure 2 is a diagram illustrating the minim level sensitivity of theinstrument; a e

1 Figure 3 is a perspective view of the gravity meter showing all of theworking elements in assembled relationship;

Figure 4 is an end elevation ofthe mass assembly: A Figurej is aperspective view of the mass assembly;

Figure 6 is an enlarged de ail fragmentary view partly in section,showing e mass clamp; v

Figure '7 is `a side elevation of the mass showing the clamping meanspartially in vertical section;

Figure a is an enlarged fragmentary perspective view showing the springguardsand the manner in which the damping means is adjustably secured tothe gravity meter housing;

Figure 9 is a plan view of one group of the spring guards;

Figure 10 is a side elevation of the group of the spring guards shown inFigure 9;

Figure 11 is a perspective view of the casting e ments in the gravitymeter and adjusting m for the pretensioned coil spring anchor;

Figure 12. is a side elevation of the support shown in Figure 11;

Figure13 is a detail sectional view of the support shown in Figure 1lshowing the slide rest locking means in their raised position `whichserves as a support for all of the operating Figure 14 is a sideelevation of the gravity meter taken with a side of the casing removedshowing all of thel working elements in their operative positions withinthe gravity meter housing;

Figure 15 is a fragmentary -perspective view of the pivot;

Figure 16 is a vertical sectional view of the nulling lever which passesthrough the housing und the support therefor;

Figure 17 is a perspective view` of the .outside of the inner gravitymeter casing showing a portion of the nulling system;

pensating for changes in temperature within the gravity meter; y

Figure 27 is another modification of the instant invention in that itshows a bellows lled with a Itemperature responsive iiuid interposedbetween the" end.I of the pretensioned coil spring and its anchor forcompensating for changes in temperature;

vFigure 28 is a detailed'sectional view showing the manner. in which thesupport is secured to and spaced from the housing;

- Figure 29 is a perspective view of the mass clamping element with theside wall to which it is secured shown-in section; and

Figure 30 is a cross section of one of the damping elements.

The general theory of the horizontal pivoted beam' can bebriefly'outlinedas follows: In Figure 1, a truly level surface isrepresented by the line PQ, while the line MN making an angle with PQ isa reference line xed to the framework of the instrument relative towhich the gravity responsive beam is rotatable. The axis of rotation ishorizontal and intersects the horizontal line PQ at 0. The center ofgravity ofthe beamis at C, a distance r from 0. 'I'he angle between is afunction of 0 rather than ur'sincethe elastic l means used to generatethe torque must obviously be anchored to the framework of the instrumentand 0 is the angle denoting the orientation of the beam relative to theframework.

For-a general-discussion no further specification of the details of theorigin and application of the elastic torque is necessary. The instantinvention as will be describedlater will first be concerned with a novelarrangement of the elastic means generating the torque -r. Forequilibrium of the pivoted beam to exist the torque must equal thegravity moment, or,

Figure 18 is a side elevation of the inner case of the gravity-meter,also showing details of the nulling system; o Figure 19 is a perspectiveview of the oven which encloses the instrument as shown in Figure 17;

Figure 20 is a vertical cross section of the oven showing the innercasing, its magnetic shield, a

layer of insulation surrounded by the heating oven, another layer ofinsulation and the outer casing;

Figure 21 is a development of the .heater coils; Figure 22 is atemperature control wiring diagram;

Figure 23 is a perspective view of the oven showing the gravity meterin` dotted lines disposed therein and the location of the thermalregulators;

Figure )24 is a perspective view of the completely assembled gravitymeter mounted in operative position on a tripod;

Figure 25 is an fenlarged fragmentary perspec- `tive view of the tripodshowing the details of constructionof the leg clamps;

Figure 26 is a modification of the instant inv vention in that it showsbi-metal means for comparameter.

From Figure '1 we' note that w.=0+, thus Ia. f=mgr cos (l'ol-l-) Thisequation is a relation that describes the angular-position (0) of thebeam as a function of gravity (g). tive of the position of thel beamwill naturally be attached to the framework of the. instrument so that 0will be the immediately observable Conditions to impose on Atheparameters of the beam to insure a detectable change in 0 will resultfrom a very small change in gravity are essenf tial for a usefulinstrument.

'I he gravity sensitivity S can be defined as the rat1o of the beamdeflection (d0) to the fractional change in gravity g producing thedeflection, or,

To evaluate S, AEquation I is digerentiated con- Any means used that isindica-A rrotation is not exactly vertical.4

sii-lering 6 and y as variable and -r as a function of 6;- thus, 1

Rearranging the terms of the above equation,-

mgr cos w g which when compared -to Equation II shows that mgr cos wIII. S

Equation III is very general and is applicable to any pivoted beam usingany type of elastic members to support the beam. Various forms of l isof the order of l0". To obtain this condition S is usually of the orderof 20 to 200 if high resolving power optical means are used to detectthe deiiections of the beam. These high values of S are readily obtainedby proper selection of the parameters in Equation III. In fact, it ispossible to cause the beam to be iniinitely sensitive to gravity bychoosing the parameters of Equation III in such a manner that thedenominator becomeszero; i. e.,

dr IV. Eg- -mgr sm w Iniinite sensitivity is of course impractical sothat in any -usable instrument S is large but not infinite. This processof securing high sensitivity by approaching the condition described byEquation IV has been described in various ways in the prior art;labilizing, astatization andinducing a long period of oscillation aremon terms that have been used.

q By a further 'consideration of EquationI it is possible to learn howthe parameters of the beam such as f, w, r, etc.,-may be chosen tominimize level sensitivity. In Figure 1 it is evident that Equation I(a) becomes Ib. r=mgr cos (H+/3) cos v l For minimum level sensitivityitis then required that4 a n da d' v approach a minimum value, .zeropossible, thereby indicating that level' errors d and d' will cause anegligible beam deectiond. I,

Diierentiating Equation 1(2)) considering 9 as a function of n Q l d"and mgr sin u cos i-mgr smw cos Since is a very small angle, cos can beapproximated by unity and by introducing the -sensitivity S the aboveequation'becomesA tall w Considering 0 as a. function of and againdifferentiating 1(1)) v1.A ddTF-s sin e' Thus minimum level sensitivityis acquired by operating the beam in such a position that w isessentially zero degrees and by maintaining the *axis of rotation in ahorizontal plane so that is essentially zero, whereupon both closelyapproach zero. This is readily accomplished by adjusting the center ofgravity of the beam into the same horizontal line (PQ) asi includes theaxis t. A level vial mounted on the framework of the instrument paralleltothe beam may be used as an indicator of the correct level position toassure that w=!l. A second level vial perpendicular to the firstmentioned vial may be used to indicate the level disposition of the axisof rotation.

A further illustration of the method of securing a minimum levelsensitivity is shown in Figure 2 wherein it is obvious that so long asthe beam is operating within a small angular range above and b'elow thehorizontal line PQ, the projection of r (the moment arm of the mass) onthe hori-` zontal plane is relatively independent of the angularposition of the beam. l

It is immediately evident that the condition for x low level sensitivitywherein w is essentially zero,

the angle represents the orientation ofA the iny strument with the truehorizontal surface. Errors in levelling will appear as variations inLevel errors in the perpendicular direction will appear as variations inthe angle which the ax'is of' rotation of the-beam makes with thehorizontal. This angle will be denoted as In the development of all theprevious equations it has been assumed that the 1axis'of rotation washorizontal and was equal to zero: To discuss level sensitivity it willbe necessary to first assume the axis of rotation-is slightly out oflevel by an amount and then to determine what effect variations in aswell as in will have on the deneetion of the beam. i

When is different from zero Equation I (a) must be modified to allow fora decrease in the gravity moment of the beam when its plane of reducesthe sensitivity equation, Equation I II to v y w To attain a highsensitivity to gravity it is now only necessary that Therefore, apreferable embodiment of any horizontal beam gravity meter will includea nulling to its horizontal position. The small eiIort exerted by thenulling means in order to restore-the beam to its preferred horizontalposition will then be a measure of the change in gravity which causedthe original deection of the beam.

Equation IV describes a relationship between the parameters of the beamwhich if satisiled resuits in an innitely sensitive beam; i. e.. a veryunstable beam. Since w=o+ it is entirely possible that some value ofwithin the narrow operating range of the beam may satisfy the conditionsof Equation IV and the beam is' unstable at a certain point which may bevery close to its preferred horizontal position where w=0. Such acondition of instability is highly undesirable in a practical fieldgravitymeter. Thus in the construction of a rugged usable gravity meterof this type the parameters of the beam must be still further'restricted to such an extent that a high beam sensitivity can beobtained while at the v same time any unstable yposition of the beamY isas far removed as possible from` the operating range of the beam.

This Vstability restriction can be readily satisned by requiring thesensitivity S to be as conmeans by which the beam can always be returnedsimple torque functions satisfying thesethree conditions are r=Z sin(,0-6)

with (0 5) in the neighborhood of 90 and Z is a constant closelyapproximating vmgr (exactly equal-to mgr when (0-6) equals 90) or with(o-n m the neignbornoodof zero degrees and again Z is a constant closelyapproximating mm' (exactly equal to mgr when (0 6) equals 0).

't is any arbitrary constant. The designer will tailed constructionswhich utilize the theory in stant as possible over theoperating range ofthe beam, i. e.,

It is obvious that 1f s is e finite constant value for all operatingpositions of the beam then there will be no positions of innitesensitivity where the beam is highly unstable. Ihe more closely yapproaches stability will be removed from the normal operating range andthe more stable the equilibrium of the beam becomes.

The new parameter `y can be evaluated directly from Equation IIIconsidering S and 0 as the variables.

In any practical instrument the conditions of minimum level sensitivityw=0 renders the iirst term on the right hand sideof the equation zeroand i 1 dir vn. ,=-s2|:1+1-nrEin i "From ths equation it is apparentthat 'y will approach zero as se olliz approaches (-mgr) zero thefurther any points of iny of the pivot for the mass Il.

From the preceding theoretlcaldiscussion it is4 y lrequired that themethod of attachment provide that the torque exerted bythe elasticmember be such a function of the angular rotation of the gravityresponsive beam that -r equal mgr (Equation I with w=0) so as to fullysupport the beam in horizontal equilibrium, that d0 I be a smallquantity compared to mgr (Equation ma) so as to provide a highsensitivity S and further that .gli

a low value of vy. Two obviously operation.

Referring to Figure 3 for the details of construction ofthe gravitymeter, there is shown a case l0, which encloses the elements of thegravity meter. Within the 'case I0 there is mounted for rotation in avertical plane a mass Il. Mass Il, shown in detail in Figures 4 and 5 issubstantially L-shaped and' hasmost of its weight concentrated at theenlarged portion l2. The outer end of the substantially horizontal massarm I3 is provided with a clamp I4 that is adapted to engage atransverse rod i5, which forms a part The outer ends -of the rod I5 -areprovided with clamps i6' that are adapted to engage one end of each' oftwo coil springs l1 (Figure 15) The opposite ends of the. coil springsIl are secured in axial alignment, by means of clamps 18, to theoperating`1 elements I9 that are rotatably and slidably mounted in theting 20. Operating elements I9 extend through t e casting and areprovided at their outer end with a knurled head 2l by means of which theI springs I1 can be positioned. In order to hold the operating'elementsin. their adjusted position, screws 22 that threadedly engage the cast#ing 20, as shown in Figure 11, are brought into isintact with the bodyof the operating elements "The'casting 201s secured to side wall 23 ofcasting 20 will not contact the side wall 23, there is provided on the`bolt 24, near the back 'face of the casting, an annular shoulder whichrests against the inner wall of. the housing l0 when the bolt is passedthrough the opening 25 in the side wall and serves to space the castingfrom the side wall. Thecasting 20 is held in adjusted position on theside wall 23 by means of a nut 26.

The vertical, arm 21 of the mass ii is provided vat its upper end with aclamp 28 by means of secured thereto.

.which one end offa pretensioned coil spring 29 is The V spring 29issecuredl'iyy a clamp-3l to an anchor plate'll by means of a screw 32.Anchor plate 3| is secured to the casting 2l through the medium -of twodovetaii slides 33 and 3.4 (Figure'll) Opposite end of the coil operateat right angles to` each other., Slide 33 can be operated in thedovetail ways 35 by means ofthe screw 36 (Figure 12) which threadedly engages the slide 33 and'is rotatably secured to the-other slide 34.Element 33 can be locked in.

adjusted position by means of the set screws 36' which press against thegib 31.

The dovetail ways 35 are formed in the second slide 34. 'I'he slide 34is adaptedgto move in dvetail ways 38 formed in the casting 26 at rightangles to the direction of movement of the slide 33. Movement of theslide 34 is eiected by means of the screw 36 that threadedly engages theslide 34. The outer end of the screw is rotatably secured-to the casting26 by means of the bracket 40 and is provided with a worm gear 4l. Wormgear 4| can be rotated to adjust the Position of slide 34 .from a pointWithout the housing by means of the worm 42 anda splined rod I43.

The slide 34 is locked f in adjusted position by means of set screws 44and 45 that are operated by means of bell cranks 46 and 4.1,respectively. The outer end of bell crank 46 is so formed that it restsupon the bell crank 41 and is adapted to be moved by the bell crank 41in one direction to loosen the set screw 44. Bell crank 41 is rotated toloosen the screw 45 by means of the end of the splined rod 43 which wheninserted in the worm 42, engages the turned-over portion 48 of the outerend of the bell crank 41 which partially oxizierlies the opening formedaxially in the worm 4 Bell cranks 46 and 41 are .biased toward lockedposition by means of springs 49 and 50, respectively. When the rod 43 ispassed through the opening of the worm 42 lits inner end engages theturned portion 48 of the bell `crank 41 in such a manner that furthermovement of the rod to insert it will cause thebell crank' 41 and bellcrank 46, carried thereby, to be raised to loosen the set screws44 and45. Further insertion of the rod 43 forces the inner end of the rod bythe turned portion 46 of bell crank 41 and thereby locks the bell cranksin their raised position. The movement of the inner end of the rod 43beyond the turned portion 46 of bell crank 41 removes the force beingexerted on the end of the rod i3 by means of the springs d3 and 5i!which would tend to throw the rod out. With the end of rod 43 extendingby the turned por` tionv48 of the bell crank 4,1, the worm liti can beturned to rotate the worm gear 4l and screw 3S to adjust the position ofslide34. When proper adjustment has been made,v in order to lock thesliding element in adjusted position, it is only necessary to remove therod 43 and the springs 4S and 5@ Will force the bell cranks downwardlyto rotate the set screws 44 and 45 to clamp the slide 34.

By means of this novel arrangement for securing the end of thepretensioned spring 29 to the casting 20, universal adjustment of theend of the spring 29 can be made to adjust either the tension in thespring itself or to adjust the lever arm through which the elasticforces exerted by the spring act on the mass.

By providing the slide 34 with means-whereby its adjustment can beeffected from a point out- -side ofthe gravity meter, the position ofthe mass can readily be adjusted without the necessity of disassemblingthe gravity meter. In this manner changes in the displacement of themass due to changes in barometric pressure or physical properties of thespring canr be readily comnensated.

The practicability of any gravity meter depends on how nearly constantthe physical properties of its elements can be maintained. There fore,in order to limit transverse vibrations that would be imparted to thepretensioned coil spring 29 when the instrument is moved there areproried by the side wall 23 of the gravity meter housing and are soarranged that their outer ends are biased away from'the coil spring 29by means of springs 58. The outer ends of the fingers 54, 55 and 56 areso formed that when adjusted to a position adjacent the coil spring theyare spaced approximately 120 apart along the circumference of thespring. VThe outer end of the fingers can be moved towardthe coil springby means of set screws 59 that threadedly engage the support 51. Fingers54, 55 and 56 do not normally contact the spring 23 but are .so ad-Justed that they will limit any transverse vibrations of the spring.

Additionally, in order to guard against chang-y engage glass inserts 64in the face 62 of the mass (Figure 4). Points 63 are formed on the endof set screws 65 that threadedly engage` thexed support 60 and extendcompletely through it to project from the face 6I thereof. In order toadjust the points 63 relative to the lfaces 6l and 62 it is onlynecessary to rotatefthe set screws 65 4'to move them in or out.

There is formed on the enlarged portion i2 of the mass a second face 66that is parallel tothe face 52 against whicha plunger 61 rests to forcethe enlarged portion l2 of the mass'downwardly against the points. 63 toclamp it. As shown. in Figures 6 and 7, the plunger 61 is in the form ofa piston which moves in the cylinder 66. Plunger n 61 is biased towardthe downward position by means of the coil spring 63. The plunger 61 isprovided with a rod 16 which extends through an opening 1lv to a pointoutside the cylinder. rlhe outer end of the rod 16 is pivotally securedto one end of a bell crank 12. Both the cylinder 68 and the bellcrank 12are carried by the sup- Dort 13 that issecured to side wall 23 of the.

housing. Bell crank 12 is pivotally mounted on the support 13 by' meansof bolt 14. The other end of the bell crank 12 is pivotally secured to alink 15 by 'means of the bolt 14. The other end of link 15 is pivotallyconnected to acap 16.

Cap 16 (Figure 29) is secured by an airtight seal to the inner end of ametallic bellows'il that extends through the side wall 23 of the gravitymeter housing. The outer end of the bellows is sealed to a iiangebushing 19 which is in turn mounted on the side wall 23. A threadedaxial hole 16 in flange bushing 16 receives a screw 11 (Figure 17)disposed within the metallic bellows. The free end of screw 11 isrotatably secured to

